This invention relates in general to electrostatography and in particular, to an improved electrostatic imaging structure.
In electrostatography, an electrophotographic imaging member, such as a plate, drum, belt or the like, containing a photoconductive insulating layer on a conductive layer, is imaged by first uniformly electrostatically charging its surface. The photosensitive imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. The electrostatic latent image is then developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image is transferred from the imaging member directly or indirectly to a support such as paper. This imaging process can be repeated many times with reusable imaging members.
The photosensitive member is provided in a variety of forms. Typical imaging members include, for example, photoreceptors for electrophotographic imaging systems, and electroreceptors or ionographic imaging members for electrographic imaging systems. Both electrophotographic and ionographic imaging members are commonly used in either belt-form or drum-form. Electrostatographic imaging member belts are seamless or seamed. The belts generally comprise a flexible supporting substrate coated with one or more layers of photoconductive material. The substrates are inorganic, such as electroformed nickel, or organic, such as a film-forming polymer. The photoconductive coatings applied to these belts are inorganic or organic. Inorganic coatings include selenium and selenium alloys. The organic photoconductive layers comprise, for example, single binder layers in which photoconductive particles are dispersed in a film-forming binder or multi-layers comprising, for example, a charge generating layer and a charge transport layer.
Electrophotographic imaging members having a belt configuration are normally entrained around and supported by at least two rollers. Generally, one of the rollers is driven by a motor to rotate the belt during electrophotographic imaging cycles. Electrophotographic imaging belts, particularly welded seam belts, are not perfectly cylindrical, tending to be slightly cone shaped. These flexible belts tend to "walk" axially along the support rollers. Belt walking causes one edge of the belt to strike one or more edge guides positioned adjacent the ends of the rollers to limit axial movement. Friction between the edge guide and the edge of the photoreceptor belt can cause wear, rip, buckle and other damage to the belt.
Belts driven around supporting rollers often slip during stop and go operations. Belt slipping becomes a serious problem when the surface contact friction between the backside of the imaging belt and the elastomeric outer surface of the drive roll is reduced as a result of aging or deposition and accumulation of undesirable foreign material on the surface of the drive roll. This slippage adversely affects registration of images, particularly where multiple, sequentially formed and transferred images must precisely register with each other in applications such as color imaging. Further, sophisticated detection systems are required with seamed belts to ensure that images are not formed on the seam. Welded belts, because of the difficulties associated with perfectly aligning overlapping ends during seam welding, are not as concentric as desired.
Supporting rollers for an electrophotographic imaging belt generally have relatively small diameters. Constant flexing of the belt around small diameter support rollers causes the seam to crack. The cracks propagate and cause belt delamination. In addition to seam cracking and delamination, dynamic flexing of the belt around the small diameter support rollers also causes cracking of the outer imaging layer. Cracking of the outer imaging layer leads to copy print defects. Further, the supporting rollers vibrate and undesirably alter the critical distances between the imaging surface of the belt and devices such as optical exposure means, charging corotrons, development applicators, transfer stations and the like.
During cyclic electrostatographic imaging processes, the anti-curl backing coating on a belt tends to wear due to frictional interaction against support rollers, the drive roller and the various skid plate backing systems. Such wear reduces the effectiveness of the anti-curl backing coating in preventing curling of edges of the belt. Moreover, as the anti-curl back coating wears, it generates dirt, debris, and other particulates. In this respect, anti-curl back coating wear adversely affects the belt operation and contaminates the image copy print-out.
Another well-known type of electrophotographic imaging member is the drum-type photoreceptor. The long term durability of drum-type photoreceptors greatly exceeds that of belt-type photoreceptors. Drum photoreceptors are coated with one or more coatings. The coatings are applied by well known techniques such as dip coating or spray coating. Dip coating of drums usually involves immersing the cylindrical drum. During the coating and subsequent drying operation, the axis of the drum is maintained in a vertical alignment. The applied coatings tend to run and, as a result, the coatings on the drum tend to be thicker at the lower end.
Coatings applied by spray coating are often uneven. Coatings having an uneven thickness do not have uniform electrical properties, thereby degrading the print quality. Coating drums in a spray batch operation is time consuming and costly. In addition, the numerous handling steps required for batch drum coating tend to increase the likelihood that one or more coatings will be damaged or contaminated. Dip or spray coated photoreceptor drums do not exhibit the superior electrophotographic characteristics of flexible electrostatographic imaging belts. Moreover during reclaiming, the coatings are difficult to remove without damaging the drum.
U.S. Pat. No. 4,068,942 (to Penwell) teaches a hollow drum and an elongated web of photosensitive material. The bulk of the material is supported inside the hollow drum while a portion of the photosensitive material is supported on the outer surface of the drum. The photosensitive web within the drum is supported by a supply roll and a take-up roll. The rolls are capable of being repositioned periodically in order to provide for the accumulation of a greater length of photosensitive web.
U.S. Pat. No. 4,400,083 (to Beisty et al.) discloses an electrostatic printer with a rotatable drum having a photoreceptor belt mounted on the drum periphery. A supply reel and a take-up reel are located in the drum interior. The drum includes a wiper mechanism for wiping the face of a cathode ray tube as the drum is rotated. The wiper includes a mechanism for periodically indexing a supply of photoreceptor web from the supply reel.
U.S. Pat. No. 4,707,712 (to Buckley et al.) teaches a method and apparatus for transporting and tensioning sheet materials in an ink jet printer. A roll of paper is stored within an imaging drum. The paper feeds through a longitudinal opening in the drum to the outside and passes around the drum, where it is held in place during the imaging process. The paper is held in place by tension provided by reverse rotation of a tensioning roller while a drive roller is locked in position.
U.S. Pat. No. 5,151,737 (to Johnson et al.) discloses a photoconductive drum with a flexible photoconductive loop and an expandable mount. The mount includes a shell with a slit allowing the shell to expand. Wedges are moved toward each other on a shaft. Cam surfaces on the wedges push against chamfered corners on ribs extending inward from the shell to expand both ends of the shell. The shell assumes the shape of the loop.
U.S. Pat. No. 5,415,961 to Yu et al., filed Sep. 29, 1992, discloses a cylindrical device comprising at least one distinct outer layer. The cylindrical device is a preformed rigid cylindrical support drum with a predetermined outer circumference. A flexible belt, with an inner circumference at least 0.5% smaller than the outer circumference of the support drum, is mounted to the drum by a process of circumferentially expanding the belt with fluid under pressure until the circumference of the inner surface of the belt is stretched to a new dimension slightly greater than the outer circumference of the support drum. The belt is then slid onto the support drum and permitted to contract to the drum outer surface by release of the fluid pressure from the supply source.